Single-photon emission computed tomography (SPECT) has become an important diagnostic tool in nuclear medicine clinics today. However, the SPECT technique is plagued with problems which limit the quality and the accuracy of quantitative information obtainable from the reconstructed image. These problems include statistical noise fluctuations, photon attenuation, collimator-detector response and scatter radiation. Extensive research efforts have been devoted in the past years to developing techniques which correct for these effects. Further improvements are still possible considering that some techniques require additional hardware and lengthy computation times, compromising their clinical utility. The goal is to provide a systematic quantitative evaluation of these corrective techniques in terms of the improvements they offer to image quality and measurement accuracy. The importance of the proposed research is in providing the scientific basis for establishing guidelines for the selection and specification of these techniques in clinical applications. This can only result in improved patient care. The specific aims of this proposal are: (1) to develop the SPECT reconstruction software, incorporating the available algorithms that correct for photon attenuation, collimator-detector response, and scatter radiation (2) to evaluate the corrective methods in terms of reconstructed image quality by means of observer performance studies and (3) to evaluate the corrective methods for their accuracy in quantitative measurements. Data from simulation, phantom studies and a few existing clinical files will be used in the proposed project. The iterative maximum likelihood method which incorporate corrections for the image degrading effects will be one of the main corrective techniques under study. In evaluating the accuracy of quantitative measurements, results from carefully designed phantom studies will be used. In evaluating image quality, two major criteria are used -- the elimination of artifacts and improvement in lesion detection, which are applied through observer performance studies.